My subscription works on my phone, but not on my tablet (or vice versa)

Access to your subscription and its functionality is linked to the account you use on your phone/tablet, and not directly to the account you are logged in with in the app itself.
Therefore, a subscription purchased via Apple can only be used on Apple devices (iPhone/iPad) and a subscription purchased via Google Play can only be used on Android devices.
For your subscription to work on both your phone and your tablet, you need to be logged in with the same account (Apple account or Google account) on that particular device.
Specifically for Android, it is also important to note: If you have multiple accounts configured in Google Play, it is important that the App is downloaded and started for the first time with the Google account the purchase was made with. Otherwise, the purchase will not be identified.